Paris Basin Anoplotherium

any modern organism (see Chapters 9-12). Just imagine land animals 10 times the size of elephants, a world with higher oxygen levels than today and dragonflies the size of seagulls, a world with only microbes, or a time when two or three different species of humans lived in Africa!

3 Climate and biodiversity change. Thinking people, and now even politicians, are concerned about climate change and the future of life on Earth. Much can be learned by studying the modern world, but key evidence about likely future changes over hundreds or thousands of years comes from studies of what has happened in the past (see Chapter 20). For example, 250 million years ago, the Earth went through a phase of substantial global warming, a drop in oxygen levels and acid rain, and 95% of species died out (see pp. 170-4); might this be relevant to current debates about the future?

4 The shape of evolution. The tree of life is a powerful and all-embracing concept (see pp. 128-35) - the idea that all species living and extinct are related to each other and their relationships may be represented by a great branching tree that links us all back to a single species somewhere deep in the Precambrian (see Chapter 8). Biologists want to know how many species there are on the Earth today, how life became so diverse, and the nature and rates of diversifications and extinctions (see pp. 169-80, 534-41). It is impossible to understand these great patterns of evolution from studies of living organisms alone.

5 Extinction. Fossils show us that extinction is a normal phenomenon: no species lasts forever. Without the fossil record, we might imagine that extinctions have been caused mainly by human interactions.

6 Dating rocks. Biostratigraphy, the use of fossils in dating rocks (see pp. 23-41), is a powerful tool for understanding deep time, and it is widely used in scientific studies, as well as by commercial geologists who seek oil and mineral deposits. Radiometric dating provides precise dates in millions of years for rock samples, but this technological approach only works with certain kinds of rocks. Fossils are very much at the core of modern stratigraphy, both for economic and industrial applications and as the basis of our understanding of Earth's history at local and global scales.

Imagine you are traveling by plane and your neighbor sees you are reading an article about the life of the ice ages in a recent issue of National Geographic. She asks you how anyone can know about those mammoths and saber-tooths, and how they could make those color paintings; surely they are just pieces of art, and not science at all? How would you answer?

Science is supposed to be about reality, about hard facts, calculations and proof. It is obvious that you can not take a time machine back 20,000 years and see the mammoths and sabertooths for yourself; so how can we ever claim that there is a scientific method in pale-ontological reconstruction?

There are two ways to answer this; the first is obvious, but a bit of a detour, and the second gets to the core of the question. So, to justify those colorful paintings of extinct mammals, your first answer could be: "Well, we dig up all these amazing skeletons and other fossils that you see in museums around the world - surely it would be pretty sterile just to stop and not try to answer questions about the animal itself - how big was it, what were its nearest living relatives, when did it live?" From the earliest days, people have always asked questions about where we come from, about origins. They have also asked about the stars, about how babies are made, about what lies at the end of the rainbow. So, the first answer is to say that we are driven by our insatiable curiosity and our sense of wonder to try to find out about the world, even if we do not always have the best tools for the job.

The second answer is to consider the nature of science. Is science only about certainty, about proving things? In mathematics, and many areas of physics, this might be true. You can seek to measure the distance to the moon, to calculate the value of pi, or to derive a set of equations that explain the moon's influence on the Earth's tides. Generation by generation, these measurements and proofs are tested and improved. But this approach does not work for most of the natural sciences. Here,

Figure 1.2 Important figures in the history of science: (a) Sir Francis Bacon (1561-1626), who established the methods of induction in science; and (b) Karl Popper (1902-1994), who explained that scientists adopt the hypothetico-deductive method.

there have been two main approaches: induction and deduction.

Sir Francis Bacon (1561-1626), a famous English lawyer, politician and scientist (Fig. 1.2a), established the methods of induction in science. He argued that it was only through the patient accumulation of accurate observations of natural phenomena that the explanation would emerge. The enquirer might hope to see common patterns among the observations, and these common patterns would point to an explanation, or law of nature. Bacon famously met his death perhaps as a result of his restless curiosity about everything; he was traveling in the winter of 1626, and was experimenting with the use of snow and ice to preserve meat. He bought a chicken, and got out of his coach to gather snow, which he stuffed inside the bird; he contracted pneumonia and died soon after. The chicken, on the other hand, was fresh to eat a week later, so proving his case.

The other approach to understanding the natural world is a form of deduction, where a series of observations point to an inevitable outcome. This is a part of classical logic dating back to Aristotle (384-322 bce) and other ancient Greek philosophers. The standard logical form goes like this:

All men are mortal.

Socrates is a man.

Therefore Socrates is mortal.

Deduction is the core approach in mathematics and in detective work of course. How does it work in science?

Karl Popper (1902-1994) explained the way science works as the hypothetico-deductive method. Popper (Fig. 1.2b) argued that in most of the natural sciences, proof is impossible. What scientists do is to set up hypotheses, statements about what may or may not be the case. An example of a hypothesis might be "Smilodon, the sabertoothed cat, was exclusively a meat eater". This can never be proved absolutely, but it could be refuted and therefore rejected. So what most natural scientists do is called hypothesis testing; they seek to refute, or disprove, hypotheses rather than to prove them. Paleontologists have made many observations about Smilodon that tend to confirm, or corroborate, the hypothesis: it had long sharp teeth, bones have been found with bite marks made by those teeth, fossilized Smilodon turds contain bones of other mammals, and so on. But it would take just one discovery of a Smilodon skeleton with leaves in its stomach area, or in its excrement, to disprove the hypothesis that this animal fed exclusively on meat.

Science is of course much more complex than this. Scientists are human, and they are subject to all kinds of influences and prejudices, just like anyone else. Scientists follow trends, they are slow to accept new ideas; they may prefer one interpretation over another because of some political or sociological belief. Thomas Kuhn (1922-1996) argued that science shuttles between so-called times of normal science and times of scientific revolution. Scientific revolutions, or paradigm shifts, are when a whole new idea invades an area of science. At first people may be reluctant to accept the idea, and they fight against it. Then some supporters speak up and support it, and then everyone does. This is summarized in the old truism - when faced with a new idea most people at first reject it, then they begin to accept it, and then they say they knew it all along.

A good example of a paradigm shift in paleontology was triggered by the paper by Luis Alvarez and colleagues (1980) in which they presented the hypothesis that the Earth had been hit by a meteorite 65 million years ago, and this impact caused the extinction of the dinosaurs and other groups. It took 10 years or more for the idea to become widely accepted as the evidence built up (see pp. 174-7). As another example, current attempts by religious fundamentalists to force their view of "intelligent design" into science will likely fail because they do not test evidence rigorously, and paradigm shifts only happen when the weight of evidence for the new theory overwhelms the evidence for the previous view (see p. 120).

So science is curiosity about how the world works. It would be foolish to exclude any area of knowledge from science, or to say that one area of science is " more scientific " than another. There is mathematics and there is natural science. The key point is that there can be no proof in natural science, only hypothesis testing. But where do the hypotheses come from? Surely they are entirely speculative?

Speculation, hypotheses and testing_

There are facts and speculations. "The fossil is 6 inches long" is a fact; "it is a leaf of an ancient fern" is a speculation. But perhaps the word

"speculation " is the problem, because it sounds as if the paleontologist simply sits back with a glass of brandy and a cigar and lets his mind wander idly. But speculation is constrained within the hypothetico-deductive framework.

This brings us to the issue of hypotheses and where they come from. Surely there are unknown millions of hypotheses that could be presented about, say, the trilobites? Here are a few: "trilobites were made of cheese", "trilo-bites ate early humans", "trilobites still survive in Alabama ", " trilobites came from the moon ". These are not useful hypotheses, however, and would never be set down on paper. Some can be refuted without further consideration - humans and trilobites did not live at the same time, and no one in Alabama has ever seen a living trilo-bite. Admittedly, one discovery could refute both these hypotheses. Trilobites were almost certainly not made from cheese as their fossils show cuticles and other tissues and structures seen in living crabs and insects. "Trilobites came from the moon" is probably an untest-able (as well as wild) hypothesis.

So, hypotheses are narrowed down quickly to those that fit the framework of current observations and that may be tested. A useful hypothesis about trilobites might be: "trilo-bites walked by making leg movements like modern millipedes". This can be tested by studying ancient tracks made by trilobites, by examining the arrangement of their legs in fossils, and by studies of how their modern relatives walk. So, hypotheses should be sensible and testable. This still sounds like speculation, however. Are other natural sciences the same?

Of course they are. The natural sciences operate by means of hypothesis testing. Which geologist can put his finger on the atomic structure of a diamond, the core-mantle boundary or a magma chamber? Can we prove with 100% certainty that mammoths walked through Manhattan and London, that ice sheets once covered most of Canada and northern Europe, or that there was a meteorite impact on the Earth 65 million years ago? Likewise, can a chemist show us an electron, can an astronomer confirm the composition of stars that have been studied by spectros-copy, can a physicist show us a quantum of energy, and can a biochemist show us the double helix structure of DNA?

So, the word "speculation" can mislead; perhaps "informed deduction" would be a better way of describing what most scientists do. Reconstructing the bodily appearance and behavior of an extinct animal is identical to any other normal activity in science, such as reconstructing the atmosphere of Saturn. The sequence of observations and conjectures that stand between the bones of Brachiosaurus lying in the ground and its reconstructed moving image in a movie is identical to the sequence of observations and conjectures that lie between biochemical and crystallographic observations on chromosomes and the creation of the model of the structure of DNA. Both hypotheses (the image of Brachiosaurus or the double helix) may be wrong, but in both cases the models reflect the best fit to the facts. The critic has to provide evidence to refute the hypothesis, and present a replacement hypothesis that fits the data better. Refutation and skepticism are the gatekeepers of science - ludicrous hypotheses are quickly weeded out, and the remaining hypotheses have survived criticism (so far).

Fact and fantasy - where to draw the line?_

As in any science, there are levels of certainty in paleontology. The fossil skeletons show the shape and size of a dinosaur, the rocks show where and when it lived, and associated fossils show other plants and animals of the time. These can be termed facts. Should a paleontologist go further? It is possible to think about a sequence of procedures a paleontologist uses to go from bones in the ground to a walking, moving reconstruction of an ancient organism. And this sequence roughly matches a sequence of decreasing certainty, in three steps.

The first step is to reconstruct the skeleton, to put it back together. Most paleontologists would accept that this is a valid thing to do, and that there is very little guesswork in identifying the bones and putting them together in a realistic pose. The next step is to reconstruct the muscles. This might seem highly speculative, but then all living vertebrates -frogs, lizards, crocodiles, birds and mammals - have pretty much the same sorts of muscles, so it is likely dinosaurs did too. Also, muscles leave scars on the bones that show where they attached. So, the muscles go on to the skeleton - either on a model, with muscles made from modeling clay, or virtually, within a computer - and these provide the body shape.

Other soft tissues, such as the heart, liver, eyeballs, tongue and so on are rarely preserved (though surprisingly such tissues are sometimes exceptionally preserved; see pp. 60-5), but again their size and positions are predictable from modern relatives. Even the skin is not entirely guesswork: some mummified dinosaur specimens show the patterns of scales set in the skin.

The second step is to work out the basic biology of the ancient beast. The teeth hint at what the animal ate, and the jaw shape shows how it fed. The limb bones show how the dinosaurs moved. You can manipulate the joints and calculate the movements, stresses and strains of the limbs. With care, it is possible to work out the pattern of locomotion in great detail. All the images of walking, running, swimming and flying shown in documentaries such as Walking with Dinosaurs (see Box 1.2) are generally based on careful calculation and modeling, and comparison with living animals. The movements of the jaws and limbs have to obey the laws of physics (gravity, lever mechanics, and so on). So these broad-scale indications of paleobiology and biomechanics are defensible and realistic.

The third level of certainty includes the colors and patterns, the breeding habits, the noises. However, even these, although entirely unsupported by fossil data, are not fantasy. Paleontologists, like any people with common sense, base their speculations here on comparisons with living animals. What color was Diplodocus? It was a huge plant eater. Modern large plant eaters like elephants and rhinos have thick, gray, wrinkly skin. So we give Diplodocus thick, gray, wrinkly skin. There's no evidence for the color in the fossils, but it makes biological sense. What about breeding habits? There are many examples of dinosaur nests with eggs, so paleontologists know how many eggs were laid and how they were arranged for some species. Some suggested that the parents cared for their young, while others said this was nonsense. But the modern relatives of dinosaurs - birds and crocodilians - show different levels of parental care. Then, in 1993, a specimen of the flesh-eating dinosaur Oviraptor was found in Mongolia sitting over a nest of Oviraptor eggs - perhaps this was a chance association, but it seems most likely that it really was a parent brooding its eggs (Box 1.1).

Box 1.1 Egg thief or good mother?

How dramatically some hypotheses can change! Back in the 1920s, when the first American Museum of Natural History (AMNH) expedition went to Mongolia, some of the most spectacular finds were nests containing dinosaur eggs. The nests were scooped in the sand, and each contained 20 or 30 sausage-shaped eggs, arranged in rough circles, and pointing in to the middle. Around the nests were skeletons of the plant-eating ceratopsian dinosaur Protoceratops (see p. 457) and a skinny, nearly 2-meter long, flesh-eating dinosaur. This flesh eater had a long neck, a narrow skull and jaws with no teeth, and strong arms with long bony fingers. Henry Fairfield Osborn (1857-1935), the famed paleontologist and autocratic director of the AMNH, named this theropod Oviraptor, which means "egg thief". A diorama was constructed at the AMNH, and photographs and dioramas of the scene were seen in books and magazines worldwide: Oviraptor was the mean egg thief who menaced innocent little Protoceratops as she tried to protect her nests and babies.

Then, in 1993, the AMNH sent another expedition to Mongolia, and the whole story turned on its head. More nests were found, and the researchers collected some eggs. Amazingly, they also found a whole skeleton of an Oviraptor apparently sitting on top of a nest (Fig. 1.3). It was crouching down, and had its arms extended in a broad circle, as if covering or protecting the whole nest. The researchers X-rayed the eggs back in the lab, and found one contained an unhatched embryo. They painstakingly dissected the eggshell and sediment away to expose the tiny incomplete bones inside the egg - a Protoceratops baby? No! The embryo belonged to Oviraptor, and the adult over the nest was either incubating the eggs or, more likely, protecting them from the sandstorm that buried her and her nest.

Figure 1.3 Reconstructed skeleton of the oviraptorid Ingenia sitting over its nest, protecting its eggs. This is a Bay State Fossils Replica.

As strong confirmation, an independent team of Canadian and Chinese scientists found another Oviraptor on her nest just across the border in northern China.

Figure 1.4 Some of the earliest reconstructions of fossil mammals. These outline sketches were drawn by C. L. Laurillard in the 1820s and 1830s, under the direction of Georges Cuvier. The image shows two species each of Anoplotherium and Palaeotherium, based on specimens Cuvier had reconstructed from the Tertiary deposits of the Paris Basin. (Modified from Cuvier 1834-1836.)

Figure 1.4 Some of the earliest reconstructions of fossil mammals. These outline sketches were drawn by C. L. Laurillard in the 1820s and 1830s, under the direction of Georges Cuvier. The image shows two species each of Anoplotherium and Palaeotherium, based on specimens Cuvier had reconstructed from the Tertiary deposits of the Paris Basin. (Modified from Cuvier 1834-1836.)

So, when you see a walking, grunting dinosaur, or a leggy trilobite, trotting across your TV screen, or featured in magazine artwork, is it just fantasy and guesswork? Perhaps you can now tell your traveling companion that it is a reasonable interpretation, probably based on a great deal of background work. The body shape is probably reasonably correct, the movements of jaws and limbs are as realistic as they can be, and the colors, noises and behaviors may have more evidence behind them than you would imagine at first.

Paleontology and the history of images_

Debates about science and testing in paleontology have had a long history. This can be seen in the history of images of ancient life: at first, paleontologists just drew the fossils as they saw them. Then they tried to show what the perfect fossil looked like, repairing cracks and damage to fossil shells, or showing a skeleton in a natural pose. For many in the 1820s, this was enough; anything more would not be scientific.

However, some paleontologists dared to show the life of the past as they thought it looked. After all, this is surely one of the aims of paleontology? And if paleontologists do not direct the artistic renditions, who will? The first line drawings of reconstructed extinct animals and plants appeared in the 1820s (Fig. 1.4). By 1850, some paleontologists were working with artists to produce life-like paintings of scenes of the past, and even three-dimensional models for museums. The growth of museums, and improvements in printing processes, meant that by 1900 it was com monplace to see color paintings of scenes from ancient times, rendered by skilful artists and supervised by reputable paleontologists. Moving dinosaurs, of course, have had a long history in Hollywood movies through the 20th century, but paleontologists waited until the technology allowed more realistic computer-generated renditions in the 1990s, first in Jurassic Park (1993), and then in Walking with Dinosaurs (1999), and now in hundreds of films and documentaries each year (Box 1.2). Despite the complaints from some paleontologists about the mixing of fact and speculation in films and TV documentaries, their own museums often use the same technologies in their displays!

The slow evolution of reconstructions of ancient life over the centuries reflects the growth of paleontology as a discipline. How did the first scientists understand fossils?